Arterial and venous endothelial cells (ECs) exhibit distinct molecular characteristics at early developmental stages. Blood vessels are diverse in size, structure and function to suit the needs of their local tissue environment. Endothelial cells, the inner lining of the blood and lymphatic vessels, have instructive roles in specifying vascular architecture and physiology. Prior to the initiation of blood circulation, ECs have already assumed specific molecular characteristics based on their arterial or venous identities. For example, in zebrafish embryos, ECs of the dorsal aorta (DA) activate a transcriptional program involving expression of notch and ephrinB2a upon stimulation by VEGF, whereas ECs of the posterior cardinal vein (PCV) express a distinct set of genes such as ephB4 and flt4 (Lawson et al., 2001; Zhong et al., 2001; Lawson et al., 2002). Similarly, in mouse embryos, capillaries of arterial origin express ephrinB2 and those of venous origin express ephB4 (Wang et al., 1998). Disrupting this arteriovenous lineage-specific expression pattern blocks circulation, highlighting the essential role for arteriovenous identity in establishing blood circulation (Gerety et al., 1999; Gerety and Anderson, 2002).
In addition to the diversity in their transcriptional profiles, ECs exhibit different cellular behaviors according to their arteriovenous origins. In zebrafish, angioblasts migrate from their lateral position to the midline in two waves to form the vascular cord. It has been hypothesized that angioblasts destined to form the DA migrate first, whereas the angioblasts destined to form the PCV migrate at a later stage (Torres-Vazquez et al., 2003; Jin et al., 2005; Williams et al., 2010). A pathway involving signaling molecules such as VEGF, Notch, PI3K and Eph/ephrin then directs a dorsal migration of ECs to form DA and a ventral migration to form PCV (Herbert et al., 2009). The diversity in lineage-dependent cellular behavior is further evident in the differential timing of angiogenesis during the formation of the dorsoventrally positioned intersegmental vessels (ISVs) in the trunk. Two waves of ISV sprouting were noted in zebrafish depending on the origin of ECs (Isogai et al., 2003; Hogan et al., 2009; Ellertsdottir et al., 2010). The first wave occurs at around 20 hours post fertilization (hpf) when ECs of the DA migrate dorsally in response to signals including VEGF and Notch to form the primary, aorta-derived vascular network. The second wave occurs about sixteen hours later (36 hpf) when a new set of vascular sprouts emerges exclusively from the PCV. Some of these secondary sprouts connect with the primary ISVs, linking the posterior cardinal vein to the primary vascular network (Isogai et al., 2003; Hogan et al., 2009; Ellertsdottir et al., 2010). The distinct timings of the arterial-derived primary sprouts and the venous-derived secondary sprouts suggest that arterial and venous angiogenesis are differentially regulated during development.
How the distinct molecular identities of arteries and veins influence lineage-specific angiogenesis has not heretofore been well-studied. The optical clarity and rapid development of zebrafish embryos, along with the fact that they are fertilized externally, offer an excellent opportunity to conduct in vivo screens for compounds modulating biological processes of interest (Zon and Peterson, 2005; Walsh and Chang, 2006), such as such lineage-specific angiogenesis.
Many diseases or conditions exist which can be benefited by a reduction in angiogenesis or a lowering of cell cholesterol levels. There is a need for new agents that can accomplish such therapeutic results.